Recent advances in the development of catalysts used for catalytic dewaxing have made it possible to produce low pour point lubricating oil base stock from waxy feeds at high yield. Dewaxing is required when highly paraffinic oils are to be used in products which need to remain mobile at low temperatures, e.g., lubricating oils, heating oils and jet fuels. The straight chain normal and slightly branched paraffinic substituents which are present in oils of this kind are waxes which cause high pour points and high cloud points in the oils. In heavy oils, cyclic paraffins and aromatics having paraffinic side chains re also present as wax. If adequately low pour points are to be obtained, these waxes must be wholly or partly removed, or converted to non-waxy analogues. In the past, various solvent removal techniques such as propane dewaxing and MEK dewaxing were used, but these techniques are costly and time consuming.
Catalytic dewaxing has been used to overcome the deficiencies of commercial solvent wax removal processes. U.S. Pat. No. 4,222,855 issued Sept. 16, 1980, to Pelrine et al. describes a process for dewaxing a waxy hydrocarbon fraction boiling between 450.degree. F. and 1050.degree. F. (about 232.degree.-566.degree. C.) using ZSM-23 or ZSM-35. U.S. Pat. No. 4,414,097 issued Nov. 8, 1983, to Chester et al. discloses dewaxing hydrocrackate with ZSM-23, where the hydrocrackate is derived from hydrocracking a hydrocarbon feedstock boiling above 343.degree. C. The zeolite used in the present process has essentially the same X-ray diffraction pattern as ZSM-23, described in U.S. Pat. No. 4,076,842. In the present invention the aluminum content is higher than previously described. The X-ray diffraction lines are quite broad due to the very small crystal size.
Catalytic dewaxing processes using conventional catalysts remove waxes by selectively cracking waxy components to produce lower molecular weight products, some of which may be removed by distillation. For example, waxy paraffins may be cracked to lighter n-paraffins, including butane, propane, ethane and methane, which do not contribute to the waxy nature of the oil. Because these lighter products are generally of lower value than the higher molecular weight materials, it would be desirable to limit the degree of cracking which takes place during a catalytic dewaxing process.
Recent advances in processes for reducing the pour and cloud points of refinery streams have provided medium pore molecular sieves for isomerization and hydroisomerzation of the paraffinic components in the refinery streams. Isomerization of wax, which converts wax to non-waxy components boiling in the same range as the wax, is preferred to the cracking reactions described above.
U.S. Pat. No. 4,734,539 discloses a method for isomerizing a naphtha feed using an intermediate pore size zeolite catalyst, such as an H-offretite catalyst. U.S. Pat. No. 4,518,485 discloses a process for dewaxing a hydrocarbon feedstock containing paraffins by a hydrotreating and isomerization process.
In U.S. Pat. No. 4,814,543 issued Mar. 21, 1989, Chen et al. teaches paraffin isomerization of a 330.degree.-650.degree. F. (about 166.degree.-343.degree. C.) boiling range feedstock having a paraffinic content with not more than 20 carbon atoms and containing at least 20 ppm nitrogen impurities. ZSM-22, ZSM-23 or ZSM-35 are suggested as alternative hydroisomerization catalysts for this application. These catalysts are taught as being resistant to nitrogen poisoning under isomerization conditions in the conversion of distillate range feedstock.
U.S. Pat. Nos. 4,689,138; 4,859,311; 4,921,594; and 5,149,421, the disclosures of which are incorporated herein by reference, teach silicoaluminophosphate molecular sieves for isomerizing waxy hydrocarbon components.
The present invention is directed to a process for dewaxing a specific type of oil, which contains wax having a low proportion of normal alkanes. Dewaxing oils of this type requires a highly selective catalyst in order to maintain a high yield of lubricating oil base stock product. The present invention provides such a catalyst. A medium pore aluminosilicate zeolite, SSZ-32, is the catalyst used in the present process. SSZ-32 has been shown in co-assigned U.S. Pat. No. 5,053,373, issued Oct. 1, 1991 to S. I. Zones, to dewax hydrocarbon feeds by selectively removing straight chain paraffins. Feeds to the dewaxing process include light gas oil, heavy gas oils and reduced crudes boiling above 350.degree. F. (about 177.degree. C.). The SSZ-32 hydrodewaxing catalyst of '373 may optionally contain a hydrogenation metal. The present process is particularly directed to heavy refinery streams containing a high proportion of non-normal paraffin waxes, and to the process for reducing the pour point of these streams at high selectivity and at high yield.